The current outbreak has killed more than 4,500 people in West Africa, most in Liberia, Guinea, and Sierra Leone

An estimated 70% of those infected have died in those countries.

Meanwhile, European Union foreign ministers are meeting in Luxembourg to discuss how to strengthen their response to the threat posed by Ebola.

European countries have committed more than 500m euros (£400m; $600m) but the UK is pressing to double that amount.

The money is being sought to help reinforce over-stretched healthcare systems in Liberia, Sierra Leone and Guinea and to mitigate the damage Ebola is doing to their economies.

Earlier, the Spanish government said a nurse who became the first person to contract Ebola outside West Africa had tested negative for the virus.

The result suggests Teresa Romero, 44, is no longer infected although a second test is required before she can be declared free of Ebola.

Ms Romero contracted the virus when treating two infected patients in a Madrid hospital.

In another development, US health officials said most of the people quarantined after coming into contact with Ebola victim Thomas Eric Duncan had been given the all-clear. The 21-day monitoring period applied to about 50 people.

Two nurses at the Texas Health Presbyterian Hospital in Dallas became infected with Ebola after treating Mr Duncan, who was from Liberia. He died on 8 October.

Nigeria praised
The WHO can declare an Ebola outbreak over if two incubation periods of 21 days pass with no new cases. The last reported case in Nigeria was discovered on 5 September, and the country is now on the verge of a clean bill of health.

The BBC's Will Ross in Lagos says that although the nation is heaving a collective sigh of relief, experts warn that Ebola will probably return to Nigeria.

The outbreak there began when Patrick Sawyer, an American-Liberian citizen, was diagnosed with the illness in July.

Nigeria declared a national public health emergency and Sawyer later died of the disease, followed by seven Nigerians.

These included Dr Ameyo Stella Adadevoh, who diagnosed Sawyer and is credited with helping to contain the outbreak at its source.

John Vertefeuille, from the US Centers for Disease Control and Prevention (CDC), said that Nigeria had taken the right steps to contain the outbreak.

"Nigeria acted quickly and early and on a large scale" he told AFP news agency. "They acted aggressively, especially in terms of contact-tracing", he added.